Silicon dioxide is the most commonly used insulator in the fabrication of integrated circuits. As the density of devices, such as resistors, capacitors and transistors, in an integrated circuit is increased, several problems related to the use of silicon dioxide insulators arise. First, as metal signal carrying lines are packed more tightly, the capacitive coupling between the lines is increased. This increase in capacitive coupling is a significant impediment to achieving high speed information transfer between and among the integrated circuit devices. Silicon dioxide contributes to this increase in capacitive coupling through its dielectric constant, which has a relatively high value of four. Second, as the cross-sectional area of the signal carrying lines is decreased for the purpose of increasing the packing density of the devices that comprise the integrated circuit, the signal carrying lines become more susceptible to fracturing induced by a mismatch between the coefficients of thermal expansion of the silicon dioxide and the signal carrying lines.
One solution to the problem of increased capacitive coupling between signal carrying lines is to substitute a material for silicon dioxide that has a lower dielectric constant than silicon dioxide. Polyimide has a dielectric constant of between about 2.8 and 3.5, which is lower than the dielectric constant of silicon dioxide. Substituting polyimide for silicon dioxide lowers the capacitive coupling between the signal carrying lines. Unfortunately, there are limits to the extendibility of this solution, since there are a limited number of insulators that have a lower dielectric constant than silicon dioxide and are compatible with integrated circuit manufacturing processes.
One solution to the thermal expansion problem is to substitute a foamed polymer for the silicon dioxide. The mismatch between the coefficient of thermal expansion of a metal signal carrying line and the coefficient of thermal expansion a foamed polymer insulator is less than the mismatch between the coefficient of thermal expansion of a metal signal carrying line and the coefficient of thermal expansion of silicon dioxide. Unfortunately, a foamed polymer has the potential to adsorb moisture, which increases the dielectric constant of the foamed polymer and the capacitive coupling between the metal signal carrying lines. One solution to this problem is to package the integrated circuit in a hermetically sealed module. Unfortunately, this solution increases the cost of the integrated circuit.
For these and other reasons there is a need for the present invention.